This invention relates to lasers.
A laser consists of a pumped gain medium situated within an optical resonator. The pumped gain medium provides light amplification, and the optical resonator provides optical feedback, such that light circulates within the optical resonator along a beam path and is repeatedly amplified by the gain medium. The optical resonator (or laser cavity) may be either a ring cavity or a standing-wave cavity. The laser cavity defines a set of longitudinal cavity modes, evenly spaced by a frequency interval referred to as the laser cavity free spectral range (FSR). Laser emission generally occurs at one or more of the longitudinal mode wavelengths. Optical pumping and electrical pumping by current injection are two known methods for pumping the gain medium. The emitted light may or may not be in the visible part of the electromagnetic spectrum.
One of the elements within the optical resonator acts as the output coupler, whereby a certain fraction of the circulating light is emitted from the optical resonator to provide the useful laser output. A partially transmitting mirror is a known output coupler. For semiconductor lasers, the output coupler is typically an end face of a semiconductor gain medium, which may be coated to provide a degree of reflectivity which optimizes performance. Semiconductor gain media typically include an epitaxially grown multilayer structure, and are classified according to the propagation direction of the emitted light. A gain medium is a surface emitter if the emitted light propagates perpendicular to the plane of the layers. A gain medium is an edge emitter if the emitted light propagates in the plane of the layers. Edge emitting semiconductor gain media typically include a single mode optical waveguide.
In order to provide tunability for a laser, or to select a specific emission wavelength of a laser, it is sometimes desirable to employ an external cavity geometry, where the laser cavity includes one or more optical elements which are spaced apart from the gain medium. The use of an external cavity for a tunable semiconductor laser allows the use of tuning elements which are difficult to fabricate in a monolithic semiconductor structure. Likewise, the use of an external cavity for a fixed wavelength semiconductor laser allows the use of wavelength selection elements which are difficult to fabricate in a monolithic semiconductor structure. For both tunable and fixed wavelength semiconductor lasers, the flexibility provided by an external cavity configuration generally provides improved optical performance (e.g. high side mode suppression ratio and improved wavelength accuracy) relative to a monolithic semiconductor laser.
In order to realize improved optical performance from an external cavity semiconductor laser, the effect of the parasitic etalon formed by the two end faces of a semiconductor gain medium must be suppressed. An intracavity etalon formed by two reflecting surfaces within a laser cavity is regarded as a parasitic if the etalon degrades laser performance. Known methods for suppressing the effect of the parasitic chip etalon include depositing anti-reflection (AR) coatings on one or both end faces of the gain chip, and orienting the gain element waveguide so that it intersects the chip end face at other than a right angle. These two methods are frequently employed simultaneously. However, even in such cases, the parasitic chip etalon frequently has an undesirable effect on laser performance.
It is therefore an object of the present invention to provide apparatus and method for reducing the deleterious effects on laser performance of parasitic etalons within an external cavity laser.
According to the present invention, laser performance is improved by appropriately matching the spectral periods of various etalons within the laser cavity. The frequency spacing between adjacent transmission peaks of an etalon is the free spectral range (FSR) of the etalon. A first embodiment of the invention is a discretely tunable external cavity semiconductor laser where a grid fixing etalon is present in the laser cavity, the grid fixing etalon FSR is a whole number multiple of the laser cavity FSR, and the grid fixing etalon FSR is a whole number multiple of the chip etalon FSR. A second embodiment of the invention is a fixed wavelength external cavity semiconductor laser where the chip etalon FSR is a whole number multiple of the laser cavity FSR, and a mode suppressing etalon is inserted into the laser cavity such that the mode suppressing etalon FSR is a whole number multiple of the chip etalon FSR. A third embodiment of the invention is a tunable external cavity semiconductor laser where the chip etalon FSR is a whole number multiple of the laser cavity FSR. A fourth embodiment of the invention is a fixed wavelength external cavity semiconductor laser where the chip etalon FSR is a whole number multiple of the laser cavity FSR.